If you pay the entwine cost, you can sacrifice the creature you gain control of with Grab the Reins.

12/1/2004

If you choose the “sacrifice a creature” mode, you choose the target of the damage on announcement, but don’t choose what to sacrifice until resolution. You must sacrifice a creature when Grab the Reins resolves, even if you don’t want to. If you don’t control any creatures at that time, Grab the Reins deals no damage.

12/1/2004

If you cast this with Entwine, you don’t get priority in the middle of resolution. You can’t steal something, do anything (like attack with it), and then sacrifice it.